In recent years, rapid progress is starting to be made in development of fuel cells for electric cars due to the success in development of a solid polymer material. A solid polymer fuel cell differs from a conventional alkali type fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell, solid electrolyte fuel cell, etc. in that it uses a hydrogen ion-selective transmission type organic membrane as an electrolyte.
This is a system which obtains electric power by using as the fuel of the solid polymer fuel cell, in addition to pure hydrogen, hydrogen gas obtained by modification of alcohol etc. and electrochemically controls the reaction with the oxygen in the air.
A solid polymer film functions sufficiently even if thin and has the electrolyte fixed in the membrane, so if controlling the dew point in the cell, it functions as an electrolyte, so there is no need to use an aqueous solution-based electrolyte, a molten salt-based electrolyte, or other medium with fluidity and enables the cell itself to be compactly and simply designed. Development work is proceeding on application for electric cars etc. As the material for forming a solid polymer fuel cell operating in a region of 150° C. or so or less, a carbon-based material is being used due to the reasons that the temperature is not that high, corrosion resistance and durability can be sufficiently exhibited under that environment, etc., but due to the issue of brittleness, it cannot be made thin and therefore it obstructs greater compactness. Furthermore, breakage-resistant carbon-based separators are also being developed, but these are expensive cost-wise. For this reason, serious R&D is being conducted on stainless steel, titanium, and titanium alloy separators able to achieve the goals in both respects.
A solid polymer fuel cell is comprised of a solid polymer film forming an electrolyte, catalytic electrode parts on the two sides of the same and comprised of carbon fine particles and precious metal superfine particles, current collectors comprised of felt-like carbon fiber composites having the functions of taking out the electric power generated there as current and simultaneously supplying reaction gas to the catalytic electrode parts (usually called “carbon paper”), separators receiving the current from there and separating the two types of reaction gas of mainly oxygen and mainly hydrogen and the cooling medium, etc. stacked together.
The configuration of a typical solid polymer fuel cell is shown in FIG. 1.
A solid polymer fuel cell 1 is composed by stacking a solid polymer film 2 forming an electrolyte, catalytic electrode parts 3 provided on this solid polymer film 2 and comprised of carbon fine particles and precious metal superfine particles, current collectors comprised of felt-like carbon fiber composites having the functions of taking out the electric power generated by the catalytic electrode parts 3 as current and supplying reaction gas comprised of mainly oxygen gas or mainly hydrogen gas to the catalytic electrode parts 3 (usually called “carbon paper 4”), and separators 5 receiving current from the carbon paper 4 and separating the mainly oxygen gas and mainly hydrogen gas.
The basic principle of a solid polymer fuel cell 1 is basically as follows: That is, in the solid polymer fuel cell 1, fuel comprised of hydrogen gas (H2) 8 is supplied from an anode side 6, passes through gas diffusion layers comprised of carbon paper 4 and a catalytic electrode part 3, forms hydrogen ions (H+), and passes through the electrolyte comprised of the solid polymer film 2. At the catalytic electrode part 3 of a cathode side 7, an oxidation reaction (2H++2e−+½O2→H2O) occurs between the hydrogen ions (H+) and the oxygen (O2) in the air 9 supplied from the cathode side 7 and water (H2O) is produced. At the time of this oxidation reaction, the electrons generated at the catalytic electrode part 3 on the anode side 6 are run through the carbon paper 4 from the separator 5 of the anode side 6 to the separator 5 of the cathode side 7, whereby current and voltage are generated between the two electrodes.
Each solid polymer film 2 is comprised of an electrolyte having a strong acidity fixed in a membrane. By controlling the dew point in the cell, it functions as an electrolyte allowing the passage of hydrogen ions (H+).
The separators 5, component members of the solid polymer fuel cell 1, separate the two types of reaction gas, that is, the air 9 at the cathode side 7 and the hydrogen gas 8 of the anode side 6, and act as channels for supplying the reaction gas and discharge the water formed by the reaction from the cathode side 7. Further, in general, the solid polymer fuel cell 1 uses a solid polymer film comprised of an electrolyte exhibiting a strong acidity, operates at a temperature of about 150° C. or less due to the reaction, and produces water, so the separators 5 for the solid polymer fuel cell are required to have corrosion resistance and durability as material properties and are required to have good conductivity and a low contact resistance with the carbon paper for efficiently conducting the current through the carbon paper 4.
The inventors have already disclosed the specific shape, ingredients, etc. for use of stainless steel as a separator or other solid polymer fuel cell member by Japanese Patent Publication (A) No. 2000-260439 and Japanese Patent Publication (A) No. 2000-256808.
However, in these, the avoidance of cracking or warping by stabilization of the working process and the further reduction of costs in measures for reducing the electrical contact resistance of the surface remained as problems for commercialization.
In stainless steel, titanium, or titanium alloy separators, the contact resistance with the carbon paper forming the current collectors is large, so causes a large drop in the energy efficiency of the fuel cell. This has been pointed out as a problem. In view of this situation, the contact resistance between the different materials used is being studied. Low contact resistance materials for solid polymer fuel cell members for obtaining the maximum energy conversion efficiency of solid polymer fuel cells are also being studied.
As such an invention, up until now Japanese Patent Publication (A) No. 10-228914 discloses a fuel cell separator obtained by press forming SUS304 to form a bulged part with a large number of surface relief shapes at the inner circumference and by forming a gold plated layer of a thickness of 0.01 to 0.02 μm at the end face at the bulged front end.
Further, Japanese Patent Publication (A) No. 2001-6713 discloses a low contact resistance stainless steel, titanium, separator, etc. for a solid polymer fuel cell characterized by depositing a precious metal or precious metal alloy on parts contacting other parts and generating a contact resistance and lowering the contact resistance with the carbon paper.
However, these all take the form of using a precious metal to lower the contact resistance. From the viewpoint of further reducing costs and saving scarce resources, a method lowering the contact resistance without using a precious metal is desirable.
Therefore, as a measure for keeping down the use of the precious metal, the technique of causing the precipitation of chrome and carbon in the stainless steel in the annealing process and conducting current through the chrome carbides exposed at the surface from a passivation film so as to lower the contact resistance has been disclosed in Japanese Patent Publication (A) No. 2000-309854. However, this method requires too much time for the annealing process of the stainless steel and is liable to reduce the productivity and to increase the costs. Conversely, if shortening the annealing time to lower the costs, metallurgically chrome-deficient layers are liable to form around the precipitated chrome carbides and lower the corrosion resistance. Furthermore, working the separator requires a strong working process, so if a large amount of chrome carbides precipitate in the metal structure before working, cracks are liable to occur during the working process.
As a method for keeping down the use of precious metals, a low temperature type fuel cell separator using stainless steel as a base material, providing a film in which compound particles are dispersed at its surface at the surface of the base material, and heating this in a nonoxidizing atmosphere to 300 to 1100° C. to break down and eliminate the film ingredients and forming SiC, B4C, TiO2, or other deposits on its surface to reduce the contact resistance and its method of production are disclosed in Japanese Patent Publication (A) No. 11-260383 and Japanese Patent Publication (A) No. 11-219713. This method requires time and effort for the process of heating in a nonoxidizing atmosphere to 300 to 1100° C. to break down and eliminate the film ingredients, so is liable to increase the costs.
Further, as a separator obtained by combining a carbon material and metal, a solid polymer fuel cell separator comprised of a metal sheet for forming the separator which is press formed etc. at the main part where the electrodes are to be positioned so as to form gas channels and which is formed with a carbon-based conductive coating layer at its front surface part is disclosed in Japanese Patent Publication (A) No. 2000-021419; a low temperature type fuel cell separator comprised of a stainless steel base material on which carbon powder is dispersed and press bonded to improve the conductivity is disclosed in Japanese Patent Publication (A) No. 11-121018; a low temperature type fuel cell separator comprised of stainless steel as a base material on the surface of which an Ni—Cr-based plated layer in which carbon-based particles are dispersed is disclosed in Japanese Patent Publication (A) No. 11-126621; and furthermore a low temperature type fuel cell separator comprised of stainless steel as a base material on the surface of which a Ta, Ti, or Ti—Ta-based plated layer in which carbon-based particles are dispersed is formed is disclosed in Japanese Patent Publication (A) No. 11-126622.
Such attempts at placing carbon at the metal side to reduce the contact resistance at the interface between the carbon paper and the stainless steel or other metal part were good tries, but the contact resistance occurring at an interface does not just occurs due to the passivation film at the metal side. The inventors discovered that in the electron structure of the interface between the carbon and metal, a pseudo Schottky barrier occurs at the carbon side and, due to this, a large contact resistance occurs. As a result of tests to reproduce this, there was the problem that a low contact resistance state could not be stably realized.
In this way, development of technology for producing metal separators utilizing the expression of corrosion resistance by a passivation film of stainless steel or titanium or titanium alloy, greatly reducing the contact resistance, and enabling even complicated working and enabling production at a low cost is extremely difficult at present.
On the other hand, it is necessary to realize a complicatedly worked shape to function as a metal separator, so the stainless steel or titanium or titanium alloy are required to have extreme workability. Therefore, if looking forward to the reduction in costs due to the future improvement of material productivity and improvement of productivity in the process of working complicatedly shaped separators, it is desirable to greatly reduce the precipitates in the metal structure obstructing material productivity and elongation at the time of working.
Therefore, for the purpose of reducing the contact resistance, a stainless steel or a titanium or titanium alloy material in which a conductive compound or metal phase is precipitated in the metal structure is disclosed in Japanese Patent Publication (A) No. 2000-309854, Japanese Patent Publication (A) No. 2004-107704, Japanese Patent Publication (A) No. 2004-156132, Japanese Patent Publication (A) No. 2004-273370, Japanese Patent Publication (A) No. 2004-306128, Japanese Patent Publication (A) No. 2004-124197, Japanese Patent Publication (A) No. 2004-269969, Japanese Patent Publication (A) No. 2003-223904, Japanese Patent Publication (A) No. 2004-2960, and Japanese Patent Publication (A) No. 2004-232074, but from the viewpoint of realizing extreme productivity and reducing the costs in the material production and working process, seen rationally, problems are believed to remain.
Due to this situation, as a practical problem, stainless steel or titanium or titanium alloy first of all basically require productivity and workability being stressed in the material design and production process design. The materials surviving this selection process are probably materials such as high workability and high productivity stainless steel like in for example Japanese Patent Publication (A) No. 2006-040608.
However, in measures for reducing the electrical contact resistance of the surface, further reduction of the cost is an issue which must be solved for commercialization.
From the above, as the treatment for a conductive surface given after working high workability stainless steel or titanium for a solid polymer fuel cell metal separator, as explained above, under the present circumstances, gold plating is recognized as the mainstream.
This current mainstream method is criticized as having problems in terms of costs or resources. Much technology is being developed to enable use of precious metals to be kept down.
For example, Japanese Patent Publication (A) No. 2003-123783 discloses a method of forming one or more types of conductive ceramic layers of TiN, TiC, CrC, TaC, B4C, SiC, WC, TiN, ZrN, CrN, and HfC at a stainless steel separator fuel electrode side.
As specific coating methods, vapor deposition or dry coating may be illustrated, but if using a vacuum apparatus etc. to dry coat these substances, there are restrictions on the film forming rate. The yield of the coated substance unavoidably drops, so the costs are liable to increase.
Further, a titanium or titanium alloy bipolar plate (separator) comprised of a base material on the surface of which conductive hard particles are dispersed and exposed by burying conductive hard particles of the M23C6 type, M4C type, or MC type, where the metal element (M) includes one or more of chrome, iron, nickel, molybdenum, tungsten, and boron, buried in the surface of the base material is disclosed in Japanese Patent Publication (A) No. 2001-357862, while a stainless steel and stainless steel separator where at least one of M23C6 type, M4C type, M2C type, and MC type carbide-based metal inclusions and M2B type boride-based metal inclusions is dispersed and exposed, where the metal element (M) includes one or more of chrome, molybdenum, and tungsten, and where the surface roughness of the stainless steel is a centerline average roughness Ra of 0.06 to 5 μm is disclosed in Japanese Patent Publication (A) No. 2003-193206.
In the latter, it is described that it is possible to form hard fine powder having this conductivity as shot.
However, in general, solid polymer fuel cells have a low output voltage per cell of a low 1V or so, so to obtain the desired output, often a large number of fuel cells are stacked and used as stacked fuel cells. For this reason, in the method of fixing hard fine powder having conductivity on the surface of a base material by shot etc., it is necessary to suppress the occurrence of warping or strain at the separators and set the conditions and perform post-treatment to obtain separators having a good flatness enabling stacking of the fuel cells, but with this method, there are the problems that the separators will deform after shaping and not be able to be stacked. Commercialization is not possible so long as the optimal conditions are not found.
Therefore, the inventors invented the method of firing a solid plating material comprised of core particles having a higher hardness than the separator and coated with a metal having a high corrosion resistance and lower contact resistance than carbon to a separator forming a fuel cell so as to forcibly deposit the metal coated on this solid plating material on the separator and disclosed it in Japanese Patent Publication (A) No. 2001-250565. Further, they discovered that by using this technique to bury a very small amount of precious metal in stainless steel or titanium or a titanium alloy, even if not covering the entire surface with a precious metal such as with gold plating, a sufficient low contact resistance is obtained and made the invention shown in Japanese Patent Publication (A) No. 2001-6713.
In this method, a precious metal is used, so without further reduction of the costs, commercialization is not possible.
Therefore, the inventors engaged in further improvements and trial and error based on the technology of Japanese Patent Publication (A) No. 2001-250565 and as a result, as disclosed in Japanese Patent Publication (A) No. 2001-89870, Japanese Patent Publication (A) No. 2003-160884, Japanese Patent Publication (A) No. 2004-76124, and International Publication WO2005/047567, invented a method of production of coated superhard particles for driving any conductive substance into a metal surface characterized by using core particles of an average particle size of 2 mm or less as cores and lightly sintering and coating these on their surfaces with fine powder of any conductive substance of an average particle size of 0.5 mm or less.
However, with this method, there was the problem that the separators deformed after shaping and could not be stacked.
As explained above, in technology for producing metal separators for solid polymer fuel cells, which are predicted on tough working processes and require extremely low costs and mass productivity, a method of securing a high productivity and high workability in the material itself and thereby realizing a process for working complicated shapes with a high productivity and using an inexpensive high productivity machining process after shaping to drive a conductive substance or conductive metal into just the member surface is most promising. In this sense, the methods and materials and members disclosed in Japanese Patent Publication (A) No. 2003-123783, Japanese Patent Publication (A) No. 2001-357862, Japanese Patent Publication (A) No. 2003-193206, Japanese Patent Publication (A) No. 2001-250565, and Japanese Patent Publication (A) No. 2001-6713 are expected to become the mainstream in the future.
However, in these as well, no improvement in the cell performance can be expected without low cost, low electrical contact resistance surface treatment.
If considering basically this technical direction, if envisioning use of conductive substances buried in surfaces in large amounts in the future, freedom from most restrictions in the amounts of resources, low cost, and above all resistance to ion release at the surface of the metal separator exposed to a corrosive environment are important keys. Further, since the blast (shot) method is used after working to mechanically drive the substances in the surfaces, it is important to finish the members into flat shapes able to withstand the process of stacking after treatment.
In regard to the point of resistance to release of ions or various types of cations from the driven-in deposits, as disclosed in Japanese Patent Publication (A) No. 2001-250565 and Japanese Patent Publication (A) No. 2001-6713, it is sufficient to drive in and bury a precious metal, but in terms of the amount of resources or cost competitiveness, the metal carbide- or metal boride-based substances disclosed in Japanese Patent Publication (A) No. 2001-357862 and Japanese Patent Publication (A) No. 2003-193206 are superior. However, in the latter, if members are exposed to the corrosive environment in solid polymer fuel cells, at least the conductive substances are liable to be corroded, release ions, contaminate the MEA (composite of solid polymer electrolyte film and electrode), and lower the power generating ability of the fuel cells.
Therefore, it is necessary to simultaneously resolve the two problems of discovering conductive substances with ion releases so small as to be able to approach precious metals and of realizing flat shapes after treating the worked separator members. Note that in the method of treatment of using the blast method after working so as to mechanically drive a conductive substance into the surface of a worked product, no approach relating to the realization of the flatness of the treated product by making the matrix metal separator worked product out of an extremely thin material has yet been invented. That is, R&D to establish indicators for quantitative evaluation and building up technical knowhow to achieve the target indicator values is becoming essential.